1. Field of the Invention
This invention relates to fluid catalytic cracking of a mixture of hydrocarbon oils and methanol in the presence of a dispersion of fine zeolite particles, exemplified by ZSM-5B. It further relates to heat balancing of the exothermic methanol reaction with the endothermic catalytic cracking reaction so that little or no addition of heat is required to maintain the catalytic reaction. It still further relates to use of a highly active catalyst, exemplified by ZSM-5B, in a very small quantity so that the catalyst is used on a once-through basis and regeneration thereof is obviated.
2. Review of the Prior Art
The use of zeolite catalysts based on rare-earth exchanged zeolites X and Y has become widely accepted by the petroleum industry for cracking gas oil to make gasoline and fuel oil. Most known or proposed processes which involve zeolite catalyzed hydrocarbon reactions utilize a fixed bed reactor with particles no smaller than 1/25th of an inch diameter or a fluidized bed of catalyst, as in FCC cracking, wherein the particle size ranges from about 1 to 140 microns with an average particle size of about 62 microns, and in all of these processes the reaction is conducted with a relatively large amount of catalyst in contact with the feed at any given instant.
A number of hydrocracking processes which have been developed involve the use of a hydrogen donor. U.S. Pat. No. 3,951,781 is an example thereof. It teaches admixing a hydrogen donor (e.g., CH.sub.3 OH, C.sub.2 H.sub.5 OH, ethers, and the like) with de-asphalted resid before fluid catalytic cracking at less than 200 psig, 1000.degree. F., and 0.5-10 seconds. The ratio of methanol to hydrocarbon charge is within the range of 0.01-5 and preferably within about 0.05-0.30 on a stoichiometric weight basis but is varied as a function of the hydrogen deficiency of a raffinate obtained from solvent deasphalting a vacuum tower bottoms, the amount of sulfur, nitrogen, and oxygen in the raffinate, the amount of polycyclic aromatics, the catalytic composition employed, and the level of conversion desired. It is significant that it is preferred to avoid any considerable or significant excess of methanol with the charge because of methanol's tendency to react with itself under some conditions.
U.S. Pat. No. 3,974,063 provides another example of a hydrogen donor process. It teaches the conversion of nitrogen-containing oil stock, such as shale oil syn crudes, by cracking in the presence of C.sub.2 -C.sub.5 hydrocarbons and/or methanol with an acidic zeolite cracking catalyst. Co-cracking of the high nitrogen stock with the carbon-hydrogen contributors provides appreciable denitrogenation of the charge, thus decreasing the need for further hydroprocessing. The desired chemical reactions are particularly promoted by a mixture of large and small pore crystalline zeolites in the presence of hydrogen donor materials and under cracking conditions within the range of 900.degree.-1100.degree. F. (482.degree.-593.degree. C.), a hydrocarbon residence time of 0.5 seconds to about 5 minutes, and a faujasite and/or mordenite crystalline zeolite within the range of 2-15 wt. %, dispersed in a suitable matrix material. The fluid cracking pressure is generally less than 100 psig. The ratio of methanol to hydrocarbon in the charge is within the range of from about 0.01 to about 5 and preferably within the range of about 0.05 to about 0.30 on a stoichiometric weight basis.
U.S Pat. No. 4,002,557 describes a process for mixing an H.sub.2 -rich, H.sub.2 -donor, or H.sub.2 -generating material, such as methanol, with gas oil, and then preheating, admixing with catalyst, and reacting in an FCC unit. It is preferred to avoid providing any considerable or significant excess of methanol with a charge because of its tendency to react with itself under some conditions.
U.S. Pat. No. 4,263,126 is directed to converting waxy or viscous hydrocarbon oils in order to lower the pour point or provide less viscous oils. Reactive dispersions of zeolites, such as HZSM-5, HZSM-11, HZSM-12, HZSM-23, HZSM-35, and HZSM-38, are used as an extremely fine powder having a particle size of less than 0.01 to about 5 microns. It is preferred to use the as-formed powder since this is convenient and least expensive if no separation or reuse of the particles is required. The reactive dispersion is heated at a temperature of about 200.degree.-500.degree. C. (392.degree.-932.degree. F.) for a time effective to induce conversion, usually from 0.1 to 72 hours.
U.S. Pat. No. 4,328,834 relates to the conversion of methanol, or a mixture of lower alcohols and related oxygenates such as ethers, aldehydes, and ketones, in the presence of a special type of zeolite catalyst maintained in an upflowing fluid condition and comprising a dispersed catalyst phase riser contact zone discharging into a more dense upflowing fluid mass of catalyst particles. The residence time in a riser contact zone is maintained at 1-10 seconds, and in the more dense phase within the range of 5 to 80 seconds. The catalyst concentration within a riser contact zone is 1-15 pounds per cubic foot and in the more dense catalyst bed phase is 20-40 pounds per cubic foot. Heat exchange means are provided to maintain the exothermic temperature rise to not more than about 195.degree. F. (90.degree. C.) while restricting the product outlet of the dense bed is restricted to within the range of 2-2.5 fluid bed of catalyst to below 800.degree. F. (427.degree. C.) and preferably not substantially above 765.degree. F. (407.degree. C.) while the reactor pressure at the bottom of the dense atmospheres. The catalyst is a zeolite having high silica-to-alumina mol ratios and high activity, even when the silica-to-alumina mol ratio exceeds 30.
Activity is generally defined in terms of an alpha value and is referred to as alpha activity which is an approximate indication of the catalytic activity of the catalyst compared to a standard catalyst. Alpha is the relative rate constant (rate of n-hexane conversion per unit volume of oxides, compositions per unit time). It is based on the activity of highly active silica-alumina cracking catalyst taken as alpha=1 (12.5% cracking at 5 minutes on stream). Alpha activity is further defined in U.S. Pat. No. 3,354,078 and the Journal of Catalysis, Vol. IV, pages 522-529, August 1965, incorporated herein by reference. The alpha scale is also described in an article published in Journal of Catalysis, Vol. VI, pages 278-287 (1966), incorporated herein by reference. The alpha scale so described will be used herein to define activity levels.
Generally, the ZSM-5 type zeolites are activated by calcining the zeolite at temperatures of about 550.degree. C. to remove water and organic templates which are typically included in the zeolite forming solution. Calcination is achieved by heating the zeolite at the desired elevated temperature in air, hydrogen, or an inert gas such as nitrogen so that the desired activity is obtained. Typically, the heat treatment proceeds for at least 1 hour, although heating may last between 1-24 hours. The heat treatment is dry (no water) although up to 3% by weight of steam may be included in the calcining atmosphere.
It is known that catalytic cracking is an endothermic process which, at relatively high reaction rates, can become thermally self-quenching. Input of heat to fluid units occurs when the catalyst is regenerated by burning off coke to create maximum temperatures of about 1350.degree. F. (730.degree. C.), the hot catalyst being then recirculated for admixture with the incoming oil. Preferably, the catalyst is accompanied by particles of an inert and attrition-resistant material or is embedded in a matrix of such inert material which acts as a heat sink for transporting heat from the regenerator to the cracking unit.
Since the discovery of highly active zeolite cracking catalysts, the use of such zeolite crystals without the accompaniment of a large amount of inert materials has long been sought as a major improvement of the catalytic cracking process. However, it has been impractical to do so without a means of supplying heat to the reactor.
A reverse difficulty has arisen with highly exothermic reactions of Fischer-Tropsch synthesis, the oxo process, and oxygenate conversions. Attempts have been made to balance such exothermic and endothermic reactions by transferring heat within surface heat exchangers, but a method for achieving a more sensitive heat balance therebetween is needed.